1. Field of the Invention
This invention relates to a battery switching device for a computer and, more particularly, to a battery switching device which includes protection means for assuring the safety of the computer system when an abnormal condition occurs in the battery switching circuit.
2. Description of Related Art
Generally a notebook type computer is provided with a battery pack and a user operates the computer by supplying power from the battery when there is no AC power supply available. A notebook computer having a liquid crystal color display normally has an operable time of about 3 hours when driven by a battery which is fully charged. However, 3 hours of operation with a fully charged battery may not be satisfactory to users.
In view of this, a notebook computer which mounts a pair of batteries and switches from one battery to the other when the voltage of the one battery decreases below a predetermined value has been proposed in Published Unexamined Japanese Patent Application (PUPA) No. 8-549679.
In addition to battery powered computers there are portable electronic devices for use by consumers in which power is supplied from a battery. As a result, protecting the battery is of a major concern in such devices because a chargeable battery is considerably expensive for consumers. In particular, various protection circuits are known which monitor the temperature of the battery and disconnect the battery when the temperature is raised to a certain degree. PUPA No. 8-64258 is an example of such a protection circuit.
In the computer having a battery switching device as disclosed in PUPA No. 8-549679, each battery is connected via switches to a common feeding point from where electrical power is solely supplied. Two field effect transistors (FETs) F1, F2) are turned on while two other FETs (F3 and F4) are turned off or both F1 and F2 are turned off while both F3 and F4 are turned on. When the switch is turned on, the other one of the batteries is connected. If, for some reason, an over current flows through the battery in use, the battery is damaged and occasionally can explode which extends the damage to the periphery. Accordingly, there is a need for protecting the battery and its surroundings. The conventional solution has been to obtain such protection by monitoring heating of the battery.
FIG. 1 shows an example of such a conventional power source of a battery switching type. A main battery and a second battery supply a DC--DC converter via field effect transistors FET1, FET2, FET3, FET4 which are switching devices. Both FET1 and FET2 are turned on while both FET3 and FET4 are turned off or both FET1 and FET2 are turned off while both FET3 and FET4 are turned on to connect one of the batteries to the converter. FET1 and FET2 are complementary to each other and connected in series as are FET3 and FET4. Therefore, the parasitic diodes of the FETs are connected in opposite directions as shown to prevent a FET which is turned off from passing a current. When a FET is damaged by over current, the source and the drain electrodes are typically short circuited. For example, when the voltage of the main battery is higher than the second battery and the main battery is connected for use while the second battery is disconnected, a short circuit occurring in FET3 causes a heavy current to flow to the parasitic diode of FET4 to overheat it. Not only is the FET destroyed by overheating but also a solder may be molten and damage the periphery. The battery is also damaged by a large current flowing therethrough.
The prior art technology does not recognize such a unique problem in a power supply of a type in which power is switched between two batteries. Because each battery is connected to a common connection point via the switches in the power supply of a battery switching type, a short circuit of a switch which is turned off caused by an over current causes a current to flow from the unused battery of a higher voltage to the battery in use of a lower voltage. Because the internal resistance of the battery is low, there is a danger that a large current is induced which not only damages the battery but also damages elements in the current path.
The causes of such a short circuit can include an over voltage, an over current and, occasionally, a defect in the switch itself. An over voltage may be induced, for example, when some set screw is loosened and rolls away to short circuit a high voltage portion and a switch, a user spills water into the computer housing to short circuit a high voltage portion and a switch, or the user drops a pin into a slot of the housing (a louver of a fan, etc.) inducing a short circuit. An over current may be induced, beside those caused by an over voltage, when a large current momentarily flows from an AC adapter to the battery when it is mounted, for example. The power source of the battery switching type may be installed in portable electronic devices, such as notebook computers, which consumers carry around. Considering that such portable electronic devices are swung around while they are carried and are used in an outdoor environment, a failure which induces an over current is likely to occur.
Because an over current brings a rise of temperature, a battery can be protected by conventionally monitoring the temperature of the battery. However, there remains a danger to elements including a switch of the switching circuit being damaged by the over current though the battery could be protected. This is because an element such as a switch overheats more quickly than the battery. When an element such as a switch is overheated, not only is the switch damaged but also solder is molten and flows out or a substrate is damaged so that the damage extends to the periphery of the switch. Once this happens, there is hardware damage and it has to be repaired in a service center or the like. In order to solve such a problem, there is a need for a switching circuit which protects itself and a battery upon an over current condition.
Overheating can be prevented by attaching the switch to a thermally conductive heat sink or an aluminum substrate to protect the switch but this does not prevent an over current per se. In addition, a heat sink or an aluminum substrate is relatively expensive and it is desired that a less expensive component be used. If a nonmetallic substrate such as a conventional epoxi substrate can be used in place of a metallic heat sink or substrate, this would contribute to a reduction in manufacturing cost and weight. Therefore, a low cost and light weight switching circuit for safely protecting it from an over current is desired.
Further, it is desired that not only the switch and the battery be safely protected when a battery switching circuit is installed in portable electronic equipment but also the size, weight and cost of the equipment are further reduced.